1. Field of the Invention
This invention relates to a planar light emitting device having a built-in light source, e.g. a light emitting diode (LED) or the like. The planar light emitting device is used as a table, sign rod or pole, backlight device for liquid crystal panel or display board, etc. It is also applicable to a vehicle meter such as a speedometer or tachometer of an automobile. Particularly, it is preferably applied to a vehicle meter that uses an LED to light up a scale, pointer or the like.
2. Description of the Related Art
A planar light emitting device is used as a sign pole and practiced into a traffic control tool having a red synthetic resin tube fitted on an end of a torch or flashlight. Such control tool turns on or flashes on and off the light to indicate that traffic is being controlled.
A planar light emitting device is also used for a display board and has a backlight device. The backlight device has a fluorescent lamp of low power consumption arranged at the back of a liquid crystal panel. The light from the fluorescent lamp is guided to the rear side of the LC panel via a synthetic resin board or glass board as a prism. However, it is impossible to obtain uniform luminous intensity distribution with the prism alone. Therefore, under the present conditions, a plurality of fluorescent lamps is disposed to get a uniform luminous intensity distribution.
A display panel such as a billboard or a display board has a fluorescent lamp at the back of a synthetic resin board of milky color. The light from the fluorescent lamp is guided to the resin board and illuminates a film disposed on the surface thereof. However, with the light of the fluorescent lamp, it is impossible to obtain uniform luminous intensity distribution with the milky resin board alone. Therefore, under the present conditions, a plurality of fluorescent lamps is disposed to get a uniform luminous intensity distribution, too. Moreover, in case of using the fluorescent lamp, it is hard to small-size the display board.
With respect to the planar light emitting device in the shape of the traffic control tool, decorative features are rarely sought in its indicator mode. Therefore, the use if restricted to the traffic control or hazard sign.
With respect to the backlight device for the display board, since plural fluorescent lamps are arranged at the back of the prism or milky resin board to make the luminous intensity uniform, the device must be large-sized. Particularly, in case the device is small-sized, the prism board of resin or glass should be mixed with a diffusing material. Then, the luminous intensity should be controlled by both the diffusing material and prism. However, in such backlight device, if a dimension and shape is changed, total design needs to be modified accordingly, thereby to increase costs. Moreover, such structure is applicable to a planar light emitter. However, it is difficult to manufacture a light emitter provided on a periphery of a rod such as the traffic control tool.
A conventional planar light emitting device with the above-mentioned structure is disclosed in Japanese Patent Publications (Kokai) No. 5-249319 and No. 6-347617.
However, a technique shown in either publication relates to a diffused light conductor having a board shape, and a scattering material is mixed in the light conductor. Consequently, attenuation factor becomes large according to a distance from a light source. Thus, it is difficult to obtain a uniform light emitting surface. For example, the publication No. 5-240319 discloses a technique to form the light conductor in a ridge shape so as to compensate the attenuation by the distance from the light source. However, it is actually hard to make uniform the luminous intensity distribution in a large area.
Generally, a magnetic type is used for a speedometer as an example of a conventional vehicle meter. The speedometer has a scale plate or dial, a pointer and a drive unit to rotate the pointer for speed indication. Namely, the pointer indicates a vehicle speed on a scale of the scale plate.
On the other hand, the vehicle meter such as the speedometer not only acts to transmit a man-machine information like the car speed, but also functions as a component of an interior of the vehicle. Then, the vehicle meter occupies a main portion of an instrument panel that is a part attracting the most attention of a driver or the like. Its design or ornamentation is important. Therefore, in recent years, many attempts are made to improve a mechanism of each kind of meter itself, an arrangement and the like, in view of diversified tastes and sensibility of users.
Moreover, it is preferable to position the vehicle meter as distant as possible from eyes of the driver, in consideration of farsightedness of middle or old aged people.
However, the conventional vehicle meter has a complicated mechanical structure, so that wear of mechanical parts should be considered in use. Thus, there is plenty of room for improvement in respect of maintenance, accuracy in indication and so on. Moreover, the design of the conventional vehicle meter is determined at the time of manufacture and cannot be changed afterwards according to the tastes of the users. Therefore, the diversified tastes of the users cannot be satisfied with the conventional design, which has the scale plate and pointer as the mechanical components. Thus, there is a room for improvement in design.
In addition, the conventional vehicle meter needs a large space for the drive mechanism. Then, it requires a considerable depth in a meter panel and cannot be thin. The design of the meter panel is limited accordingly, and there is a room for improvement in an aesthetic point.